Method for producing a hard metal stock

ABSTRACT

The invention relates to a process for producing a hard metal batch by drying a wet slurry - 3 - which is produced by wet-milling the hard material and binder metal fractions desired in the finished batch to the desired grain size, using water as liquid phase.  
     According to the invention, the wet slurry - 3 - is applied to a moving carrier belt - 1 - with a solids content of from 75% by weight to 95% by weight and in a layer thickness of from 0.2 mm to 2 mm. As it passes through a drying zone - 4 -, the wet slurry - 3 - is heated, over the course of in total from 1 minute to 7 minutes, to a maximum temperature in the range from more than 100° C. to 150° C. The time which it takes for the wet slurry - 3 - to be heated to more than 100° C. is in a range from 15 seconds to 2 minutes. The hard metal batch which has been dried in this manner is then cooled to room temperature and if necessary comminuted.

The invention relates to a process for producing a hard metal batch by drying a wet slurry produced by wet-milling the hard material and binder metal fractions to the desired grain size, using water as liquid phase, with or without fractions of a pressing aid.

Shaped parts formed from hard metal alloys are produced by pressing and sintering a hard metal batch. The hard metal batch contains the hard material and binder metal fractions desired in the finished hard metal alloy in finely distributed form, with or without the use of a pressing aid. In many cases to produce a hard metal batch of this type, the fine-particle starting powders with a mean grain size in the range of a few μm, and in some cases even smaller, are converted into granule form, i.e. into as ideal a spherical shape as possible, with a granule size of approximately 140 μm. This improves the flow properties of the hard metal batch, which in particular significantly simplifies the uniform filling of the press moulds for production of shaped parts of complex shape.

The granules are produced by spray-drying the desired hard metal batch in a spray-drying installation. A process of this type using water as liquid phase is described, for example, in Austrian utility model AT U 4.929. A drawback of a process of this type is that it is relatively expensive. Good flow properties of the hard metal batch are not necessarily required for many shaping processes for further processing of a hard metal batch. These include, for example, the production of simple shaped parts by hydrostatic compacting or extrusion, but also the production of complex small shaped parts by powder injection molding.

In all processes for producing a hard metal batch, it is important to achieve a maximum residual moisture content in the range of <0.25% by weight and that the oxygen content in the batch does not exceed 1.2% by weight.

Therefore, the object of the present invention is to provide a process for producing the hard metal batch for hard metal shaped parts whose production does not require good flow properties on the part of the hard metal batch which is significantly less expensive than previously known processes.

According to the invention, this is achieved by virtue of the fact that the wet slurry is applied to a moving carrier belt with a solids content of from 75% by weight to 95% by weight and a layer thickness of from 0.2 mm to 2 mm and as it passes through a drying zone is heated, over the course of in total 1 minute to 7 minutes, to a maximum temperature within a range from >100° C. to 150° C., the time which it takes for the wet slurry to be heated to over 100° C. being within a range from 15 seconds to 2 minutes, and that the hard metal batch which has been dried in this manner is cooled to room temperature and if necessary comminuted.

This allows particularly inexpensive production of a hard metal batch under air and at standard pressure conditions, and this batch can be successfully processed further by simple shaping processes, such as for example hydrostatic compacting or extrusion. For further processing of the hard metal batch, in particular by extrusion, it may be expedient for a pressing aid to be admixed to the wet slurry before it is dried. If this pressing aid is a water-insoluble pressing aid based on wax, such as for example paraffin, it is admixed to the wet slurry in the form of an emulsion which is produced with the aid of an emulsifier and the addition of water.

On account of the careful setting of solids content of the wet slurry, layer thickness, drying time and maximum temperature during the drying operation, the process according to the invention, despite the use of water as liquid phase and despite the drying in air at elevated temperatures, surprisingly allows the production of hard metal batches with an extremely low oxygen content. The oxygen content is under certain circumstances even lower than in hard metal batches which are produced in accordance with the prior art and in which the wet slurry is prepared using organic solvents, such as acetone, followed by drying in vacuo. The maximum drying temperature and the drying time are matched to the layer thickness of the wet slurry applied. The greater the layer thickness, the higher the maximum drying temperature and the longer the drying time required.

To produce hard metal batches with very fine-grain hard material powders, which require significantly longer milling times, it is advantageous for an antioxidant, for example based on amino compounds, e.g. aminoxethylate or resorcinol, to be added to the water prior to milling for producing the wet slurry, with the result that an excessive oxygen content in the dried batch is prevented even with these oxidation-sensitive hard metal batches.

The process according to the invention works particularly advantageously if the wet slurry is applied to the carrier belt in a layer thickness of from 0.5 mm to 1 mm, since the total time taken to pass through the drying zone can then be shortened to 1.5 minutes to 6 minutes, and the time taken for the wet slurry to be heated to over 100° C. can be restricted to a range from 30 seconds to 60 seconds. This minimizes the oxygen uptake by the hard metal batch.

It is particularly expedient for the wet slurry to be heated to the desired temperature first of all by hot air and then additionally by radiant heat, resulting in particularly rapid removal of the moisture from the wet slurry.

The radiant heat is in this case advantageously generated by infrared radiators.

The text which follows provides a more detailed explanation of the invention on the basis of production examples and with reference to a drawing, in which:

FIG. 1 shows an outline diagram of a belt drying installation for carrying out the production process according to the invention.

EXAMPLE 1

To produce a hard metal batch consisting of 6% by weight of cobalt, 0.4% by weight of vanadium carbide, remainder tungsten carbide, 36 kg of cobalt powder with a Fisher mean grain size of approximately 0.6 μm and an oxygen content of 0.56% by weight, 2.4 kg of vanadium carbide powder with a Fisher mean grain size of approximately 1.2 μm and an oxygen content of 0.25% by weight, and 563.5 kg of tungsten carbide powder with a BET surface area of 1.78 m²/g, which corresponds to a Fisher mean grain size of approximately 0.6 μm, and an oxygen content of 0.28% by weight were milled with 100 l of water for 5 hours in an attritor.

The milling bodies used were 2000 kg of hard metal beads with a diameter of 9 mm. The rotational speed of the attritor was 78 rpm, and the wet slurry was circulated at a pumping rate of 1000 l/hour. The temperature of the wet slurry during milling was kept constant at approximately 40° C. The fully milled wet slurry had a viscosity of 4000 mPas at a shear rate of 5.18 [1/s].

A belt drying installation which operates in air and at standard pressure as shown in FIG. 1 was used to dry the wet slurry produced in this manner. The belt drying installation shown in FIG. 1 comprises a 3 m long, revolving conveyor belt -1-. A feed device -2- for applying the wet slurry -3- to the conveyor belt -1- in different layer thicknesses is provided at the start of the conveyor belt -1-. This feed device is followed by a drying zone -4-. As it passes through this drying zone -4-, the wet slurry -3- which has been applied in layer form is heated in a first passage zone -5- by means of a hot air blower -6-.

In a subsequent second passage zone -7-, the pre-dried wet slurry is heated to the desired maximum temperature within a range from >100° C. to 150° C. by means of infrared radiators.

At the end-side turning point of the conveyor belt -1-, the dried hard metal batch drops into a collection vessel -8-.

In the present production example, the belt drying installation was operated at a belt velocity of 1 m/min, and the wet slurry -3- was applied to the conveyor belt -1- in a thickness of 0.8 mm. The total passage time through the drying zone -4- was 3 minutes, with the following temperature sequence: after 20 seconds: 40° C. after 52 seconds: 50° C. after 84 seconds: 60° C. after 150 seconds: 95° C. after 165 seconds: 102° C. after 180 seconds: 110° C.

Even as it was passing through the first passage zone -5-, the applied wet slurry -3- was heated to a temperature of approximately 60° C. by the hot air blower -6-, with the result that the majority of the water was evaporated. As a result, it was possible for the time of the second passage zone -7-, in which the wet slurry was at a temperature of more than 100° C., to be restricted to approximately 15 seconds in order to achieve the required maximum permissible residual moisture content in the dried hard metal batch.

Then, the dried hard metal batch was cooled to room temperature over the course of 20 seconds. The oxygen content of the hard metal batch dried in this manner was 0.53% by weight, and the residual moisture content was 0.13% by weight.

The dried hard metal batch was broken up in a hammer mill, to a particle size of approximately 0.4 mm, mixed with a plasticizer in the standard way and extruded to form a hard metal rod with a diameter of 16 mm. This rod was then sintered for 80 minutes at 1410° C. and then recompacted at 70 bar.

Metallurgical examination revealed an excellent quality of hard metal with the following properties: Density: 14.83 g/cm³ Hardness HV30: 2.035 daN/mm² Magnetic saturation: 114 × 10⁻³ T m³/kg Coercive force: 485 Oe.

For comparison purposes, two further hard metal batches of the same composition as in Example 1 were produced. In Example 2, the hard metal batch was produced by spray drying, and in Example 3 the hard metal batch was produced by vacuum drying with organic solvent. The hard metal alloys produced from these hard metal batches were then compared with one another.

EXAMPLE 2

To produce this hard metal batch, the same raw materials in the same ratio and the same quantities as in Example 1 were milled with 160 l of water, under otherwise identical conditions, in an attritor.

A spray tower with a cylindrical section 6 m high with a diameter of 4 m was used to dry the wet slurry produced in this manner. The spray tower was designed for countercurrent operation in accordance with the fountain principle.

The gas used to dry the wet slurry was air which was fed to the spray tower at 4000 m³/hour. The wet slurry was fed to the spray tower via a spray lance with a single-flurry nozzle having an outlet opening with a diameter of 1.12 mm, at a pressure of 15 bar. The air outlet temperature was set to a constant level of 88° C., which under the given conditions was achieved by an air entry temperature of 145° C.

The spray-dried hard metal granules produced in this way, with a mean grain size of 125 μm, had an oxygen content of 0.52% by weight and a moisture content of 0.15% by weight.

The hard metal granules produced in this way were mixed with a plasticizer in the standard way and extruded to form a hard metal rod with a diameter of 16 mm.

This rod was then sintered for 80 minutes at 1410° C. and then recompacted at 70 bar.

The metallurgical assessment revealed a hard metal quality having the following properties: Density: 14.85 g/cm³ Hardness HV30: 2030 daN/mm² Magnetic saturation: 112 × 10⁻³ T m³/kg Coercive force: 491 Oe.

EXAMPLE 3

To produce this hard metal batch, the same raw materials in the same ratio and the same quantities as in Example 1 were milled for 8 hours in an attritor filled with 135 l of acetone.

The temperature of the acetone/powder mixture during the milling was kept constant at approximately 35° C.

The fully milled suspension had a viscosity of <200 mPa at a shear rate of 5.18 [1/s].

A vacuum drier of conventional design was used to dry this hard metal suspension produced in this way.

In this case, the heat was supplied via hot water in a double jacket. Moreover, vacuum was applied to the suspension, so that the acetone evaporated. In addition, this vacuum drier was equipped with a slowly rotating stirring mechanism.

After a drying time of 10 hours, the hard metal batch produced in this way had an oxygen content of 0.48% by weight. The hard metal batch produced in this manner was pressed through a screen then mixed with a plasticizer in the standard way and extruded to form a hard metal rod with a diameter of 16 mm. This rod was then sintered for 80 minutes at 1400° C. and then recompacted at 70 bar.

The metallurgical assessment revealed a hard metal quality with the following properties: Density: 14.83 g/cm³ Hardness HV30: 2032 daN/mm² Magnetic saturation: 113 × 10⁻³ T m³/kg Coercive force: 488 Oe. 

1-5. (canceled)
 6. A process for producing a hard metal batch, which comprises the following steps: providing a wet slurry with a solids content of from 75% by weight to 95% by weight and water as a liquid phase; depositing the slurry on a moving carrier belt at a layer thickness of from 0.2 mm to 2 mm; passing the slurry through a drying zone and heating the slurry in the drying zone, for a time period of between 1 minute and 7 minutes, to a maximum temperature within a range from >100° C. to 150° C., wherein a time period during which the slurry is heated to over 100° C. is within a range from 15 seconds to 2 minutes, to form a dried hard metal batch; and cooling the hard metal batch to room temperature and, optionally, comminuting the hard metal batch.
 7. The process according to claim 6, which comprises forming the wet slurry by wet-milling hard material and binder metal fractions to a desired grain size, and optionally adding fractions of a pressing aid.
 8. The process according to claim 6, which comprises depositing the wet slurry in a layer thickness of from 0.5 mm to 1.0 mm.
 9. The process according to claim 8, which comprises passing through the drying zone within a total time period of from 1.5 minutes to 6 minutes, setting the maximum temperature to 120° C., and heating the wet slurry to over 100° C. within a time range from 30 seconds to 60 seconds.
 10. The process according to claim 6, which comprises first heating the wet slurry to a desired temperature by hot air and subsequently heating by way of additional radiant heat.
 11. The process according to claim 10, which comprises generating the radiant heat by infrared radiators. 